This invention relates to a television tuning system and more particularly to a system for enabling a counter which identifies the channel to which a radio frequency tuner is tuned.
A tuning system which provides for the selection of any one of a plurality of television channels in, for example, three bands of radio frequencies is disclosed in my copending application Ser. No. 476,081, now U.S. Pat. No. 3,924,192, filed concurrently herewith and entitled, "Random Channel Address Crystal-Lock Tuning System." To access a given channel, an oscillator of an appropriate tuner for tuning one of the three radio frequency bands is swept throughout a band of oscillator frequencies. As the oscillator is swept, the oscillator signal is compared to a plurality of harmonically related frequency samples which are spaced apart by a frequency related to a regular channel spacing. A counter accumulates a count representative of the number of frequency comparison conditions encountered during a period of sweeping of the oscillator. To obtain an accurate count in the counter, a counter enable system is provided to allow the counts to begin entering the counter only after reaching a predetermined point in the sweeping process.
One approach which has been suggested to establish the point at which the counter will start to accumulate counts during the sweeping process involves providing a reference voltage which is compared to the voltage generated to tune the oscillator of the tuner. When the generated voltage is equal to or has a fixed relationship to the reference voltage, the counter is enabled and counts are accumulated therein. With such a system, the reference voltage must be adjusted individually in each receiver to accommodate the variations in tuning voltage versus frequency characteristics from tuner to tuner. Also, the variations of tuner frequency tuning voltage versus frequency characteristics with temperature change must be carefully controlled to prevent inaccurate accumulation of counts in the counter caused by a shift in temperature in the environment of the tuner.